Bores are drilled from the surface for various purposes, such as exploration and production in the oil and gas industry. Bores may be formed with drills of an initial diameter and subsequently enlarged. For example, an initial narrow bore may be made when all characteristics of the formation to be drilled are not known, such as whether gas could be encountered whilst drilling. Upon completion of the narrow borehole, a desired greater final bore diameter may be safely achieved by a reaming operation. In other instances the initial bore diameter may be less than desired, such as due to wear of a drill bit, or encroachment by material adjacent the bore (e.g. plastic formation creep).
Bores are often lined, such as with sections of casing in the oil and gas industry to provide a continuous fluid-tight conduit for conveying fluids through the bore. Typically the casings are fixed in place by injecting cement in an annular space between the casing and the bore wall. Once a first section of bore has been lined, a subsequent section is bored, involving passing the drill bit through the casing. Consequently the drill bit has a smaller diameter than the casing and in order to drill a bore at a larger diameter, such as the same diameter of the first section of bore, an under-reamer is often mounted above the drill bit.
In addition to continuous sections of larger diameter, reamers may be used to create localized sections of larger diameter. For example, where a feature in a bore is to be placed, such as a branch or deviation, the bore diameter may be focally enlarged.
Tools used for reaming operations comprise extendable cutters that are actuated to project out from the diameter of the main body of the tool to enlarge the bore diameter. Typically the cutters are also retractable for subsequent passage through a narrower section, such as for retrieval through casing. The extent to which the cutters can extend limits the maximum diameter that can be reamed. In order to increase the maximum reamable diameter, hinged cutters are used; however the hinged constructions are relatively weak and the hinged cutters can fail leading to costly interruptions and retrieval operations.